This invention relates to an apparatus and method for the fractional distillation of liquid mixtures.
Fractional distillation is widely employed for the separation of liquid mixture components (i.e. hydrocarbons) of different boiling points. Such fractional distillation is normally carried out in a fractionation column. The feed material is usually fed into the approximate mid-section of the column, the feed material thereafter separating into a vapor phase and a liquid phase. The vapor phase passes out of the upper section of the column and a portion of the vapor phase is condensed and returned to the column as reflux. The liquid phase flows down the column through a plurality of trays, and at least a portion of the liquid phase is withdrawn from the column bottom as the bottoms product. Another portion of the liquid which accumulates at the column bottom is withdrawn, heated in a reboiler, and returned to the column.
A problem which arises in fractionation columns for fractional distillation of oils involves cold liquids from the bottom tray bypassing the reboiler and being withdrawn with the bottoms product. The bottom tray liquids contain a greater concentration of light hydrocarbon impurities than the reboiler return liquids. Thus, bottom tray liquids which are yielded directly as bottoms product without passing through the reboiler raises the light hydrocarbon impurity level of the bottoms product. This must be compensated for by overheating liquids passing through the reboiler so as to waste a considerable amount of energy.
One attempt to solve the above problems is described in U.S. Pat. No. 2,795,536 of Grossberg et al. Grossberg et al employs a vertical baffle plate to assist in directing cool liquid from the bottom tray to a pool from which liquid is withdrawn and passed to a reboiler. This insures that the liquid from the bottom tray passes through the reboiler at least once before being yielded out with bottoms product.
More specifically, Grossberg et al discloses the use of a downspout plate for receiving a flow of liquid therealong from a bottom tray. A cup member is provided in conjunction with the lower end of the downspout plate to provide the desired liquid seal to prevent gases passing up and around the downspout plate. The orientation of the above mentioned vertical baffle plate and the construction of features, such as the cup member, associated therewith require that a separate baffle plate must be provided for each cup member and associated downspout plate to achieve the above discussed result wherein substantially all liquids coming directly from the bottom tray are prevented from being passed out of the column as bottoms product. Thus, in a multipass system a number of baffle plates are required which increases expense and complexity of construction.